IP Passthrough vs. Bridge Mode in ISP Gateways and Client Firewalls

When configuring an ISP-provided gateway to work with a client firewall or router, two commonly used modes are IP Passthrough and Bridge Mode. Understanding these modes is crucial for setting up a network correctly, ensuring proper routing, and avoiding double NAT issues.

1. IP Passthrough

Definition

IP Passthrough is a feature commonly found in ISP-provided routers or modems that allows a single device (typically a firewall or a separate router) to receive the public IP address assigned by the ISP. This effectively makes the ISP gateway act as a simple modem, forwarding all traffic to the designated device without performing NAT (Network Address Translation).

How It Works

• The ISP assigns a public IP address to the ISP gateway.
• With IP Passthrough enabled, the gateway forwards that public IP address directly to a single connected device (usually the client firewall or router).
• The device receiving the public IP is responsible for routing and firewall functions.
• The ISP gateway itself continues to have an internal private IP for management purposes but does not route traffic.

Use Case

• When a business or advanced home user has a dedicated firewall (e.g., a Cisco ASA, Fortinet, Palo Alto, or pfSense firewall) and wants full control over network security and routing.
• When avoiding double NAT (Network Address Translation), which can cause connectivity issues with VPNs, VoIP, and certain applications.

Key Considerations

• Typically, only one device can receive the public IP.
• Some ISPs limit the ability to use IP Passthrough unless the customer has a static IP or a business account.
• If the ISP changes the public IP dynamically, the firewall must be configured to handle DHCP-assigned WAN IPs.
• Some ISP routers may still interfere with certain traffic (e.g., block ports, impose firewall rules) even in passthrough mode.

2. Bridge Mode

Definition

Bridge Mode is another configuration method that disables the ISP gateway’s routing and NAT features, making it function purely as a pass-through device (essentially a modem). This mode allows the customer's firewall or router to take full control over network operations.

How It Works

• The ISP gateway disables all routing features, including NAT, DHCP, and firewall functions.
• The firewall or router directly handles the public IP address assignment, routing, and security policies.
• All network traffic flows transparently between the ISP and the client network.

Use Case

• When a static IP block (multiple public IPs) is provided and must be assigned to different devices inside the network.
• When a business-class firewall is used that requires complete network control.
• When using a third-party router instead of the ISP’s equipment.
• In scenarios where multiple public IP addresses need to be assigned to different internal devices.

Key Considerations

• Not all ISP gateways support full bridge mode. Some still retain limited firewall rules or DHCP capabilities.
• If misconfigured, the ISP router could cause double NAT, leading to performance and connectivity issues.
• With bridge mode enabled, the ISP router loses all routing capabilities, meaning all routing and firewalling must be done on the client-side device.
• In some cases, customer support from the ISP may be limited if bridge mode is enabled.

3. Dynamic IP vs. Static IP in the Context of ISP Gateways and Firewalls

Dynamic IP Address

• Assigned automatically by the ISP (via DHCP).
• Changes periodically (e.g., daily, weekly, or when the modem reconnects).
• Often used for residential and small business customers.
• Requires Dynamic DNS (DDNS) services if hosting public-facing services (e.g., VPN, web server).

Static IP Address

• Fixed, does not change (unless manually reassigned by the ISP).
• Required for businesses running servers (e.g., mail servers, web servers, VPNs).
• Essential for remote access, site-to-site VPNs, and VoIP systems that require consistent addressing.
• Usually an additional cost from the ISP.
• If using a block of static IPs, they can be assigned in Bridge Mode to different internal devices.

Comparison Table: IP Passthrough vs. Bridge Mode

Conclusion

Both IP Passthrough and Bridge Mode allow better integration between an ISP gateway and a client’s firewall/router, but they serve slightly different purposes.

• Use IP Passthrough if you only need a single public IP on your firewall/router while keeping the ISP gateway operational for basic connectivity.
• Use Bridge Mode if you have a static IP block or need complete control over routing, and your ISP supports full bridge functionality.

Proper configuration depends on ISP limitations, network needs, and whether you are using a dynamic or static IP.

Provide thorough definitions and overview of IP passthrough and bridge mode as it relates to ISP Gateways and client firewalls. Include related information on dynamic IP vs static IP.